As described in Patent Literature 1, a known type of step-up/down DC-DC converter keeps the output voltage constant irrespective of variation in input voltage. Such a step-up/down DC-DC converter includes a voltage decreasing switch and a voltage increasing switch. In a voltage decreasing mode, the voltage decreasing switch is switched while the voltage increasing switch is kept off. In a voltage increasing mode, the voltage decreasing switch is kept on while the voltage increasing switch is switched. In a voltage increasing/decreasing mode, the voltage decreasing switch and the voltage increasing switch are switched alternately.
The step-up/down DC-DC converter described above gradually reduces the number of times that the voltage decreasing switch is switched in the changeover from the voltage decreasing mode to the voltage increasing/decreasing mode. It gradually increases the number of times that the voltage decreasing switch is switched in the changeover from the voltage increasing/decreasing mode to the voltage decreasing mode. Similarly, the step-up/down DC-DC converter gradually reduces the number of times that the voltage increasing switch is switched in the changeover from the voltage increasing mode to the voltage increasing/decreasing mode. It gradually increases the number of times that the voltage increasing switch is switched in the changeover from the voltage increasing/decreasing mode to the voltage increasing mode. In this manner, the step-up/down DC-DC converter changes switching operations of the voltage decreasing switch and the voltage increasing switch smoothly when the modes are changed, so that the output voltage is kept constant.
As described above, the step-up/down DC-DC converter described in Patent Literature 1 gradually reduces or increases the number of times that the voltage decreasing switch or the voltage increasing switch is switched in a mode changeover. Although this method allows the step-up/down DC-DC converter to keep the output voltage constant, the method poses a problem that the frequency band of noise generated by the switching increases.